This invention relates to two-wire transmitters that operate in conjunction with a current loop. The loop includes an external power supply, a pair of wires from the supply, and the transmitter connected serially between the wires. An external sensor also connects to the transmitter. In operation the transmitter energizes the sensor, and in response it receives information signals from the sensor. This information is transmitted on the pair of wires by varying the current loop in the current loop. That is, the transmitter acts as a variable current sink; and the amount of current which it sinks is representative of the information from the sensor.
Due to various industry standards, two-wire transmitters must operate under certain constraints. One of these constraints is that the loope current can vary only between four milliamps and twenty milliamps. This insures that the other devices in the loop, such as recorders, will properly interpret the loop current signal. Another constraint, is that the external power supply must have a relatively small voltage output. Typically, a loop supply voltage of 28 volts DC is used. This limitation is imposed for safety reasons. As a result, the amount of power which the two-wire transmitter may draw from the current loop to use for its operation is severely limited. In fact, various types of sensors exist which require more power for the excitation than can be drawn from the loop. Accordingly, two-wire transmitters could not in the past be used with these type of sensors.
A standard strain gage is one sensor, for example, which falls into this category. The strain gage is comprised of four resistors which are interconnected to form a bridge. Typically, the bridge resistance is 100 ohms. Thus, when four volts is used to excite the bridge, power dissipated therein is voltage squared divided by resistance or 160 milliwatts. This amount of power can, of course, be made less by applying a smaller excitation voltage; however, bridge sensitivities are small, with 3 millivolts per volts being very common. Thus, a large excitation voltage is desirable in order that the output from the bridge have a reasonable signal to noise ratio.
In addition to power being dissipated in the sensor, power is also dissipated in the two-wire transmitter itself. However, even if we assume that no power is dissipated in the transmitter, a 100 ohm bridge with four volts of excitation would not be feasible due to the above described industry standard power constraints. Suppose for example, the signals from the sensor were such that the transmitter was required to sink only four milliamps. Simultaneously of course, the transmitter must receive the 160 milliwatts of power from the loop. Thus, the voltage drop required across the transmittter would be 160 milliwatts divided by 4 milliamps or 40 volts. This of course, is greater than the 28 volts that is available from the external supply. Prior art two-wire transmitters do not solve this problem.
Therefore, it is one object of the invention to provide an improved two-wire transmitter.
Another object of the invention is to provide a two-wire transmitter which excites an external sensor at a power level which is greater than the power level which the transmitter can continuously draw from the two-wire loop.